front derailleur compatability issues

I currently have a (frame) damaged TREK 1200, vintage approx. 2000, whose components I would like to be able to switch to a new TREK 1500 frame. The problem I need help with is the front derailleur.

My current set up is a clamp-on front Shimano RSX derailleur with a triple Shimano SG A 52-42-30 chainring. The set up in the rear is an 8-speed rear cassette (26T - 13T) on a Shimano RSX rear hub FH-A416. It is a Shimano RSX derailluer in the back. The shifters are of the double tap variety.

The problem is with the seat tube that I have to clamp the derailleur on. The 1200 seat tube is 27.1 mm and the 1500 is larger at 31.8 mm. I can't machine the clamp to accommodate the larger diameter as it would remove too much material. A new front derailleur seems inevitable. Went to the LBS and they think I am going to be out of luck due to compatibility issues (these days most of the rear cassettes are 10 speed clusters). They mention something about probable chain rub up front.

I am not sure how to best describe the chain, but when I measure the width I get a max outside width of 7.07 mm at the rivets, 6.73 mm at the outer links and 4.30 mm at he inner links.

Does anyone know of a compatible front derailleur or other (non-expensive) fix for this situation?

I could be wrong, but triple front derailluers tend to be wider, but if you need an older 8 speed, look on ebay. it seems that most 8 speed clamp ons are going to be 28.6, if you need 31.8, you might want to get a braze on and you could get a separate braze on clamp. good luck.

Front derailleur clamps and clamp-on adapters for braze-on type derailleurs come in three diameters of 1-1/8" (28.6 mm), 1-1/4" (31.8 mm) and 1-3/8" (34.9 mm) so your frames are one of these three sizes.

I have a very lightly used 105 braze-on 8-speed triple front derailleur intended for a 52/42/30 crank. It's yours for the asking. PM me if you want it

You will need an adapter clamp of the correct diameter but any LBS should have them and Nashbar offers them at a very low cost.

Praises for forum members

Originally Posted by HillRider

Front derailleur clamps and clamp-on adapters for braze-on type derailleurs come in three diameters of 1-1/8" (28.6 mm), 1-1/4" (31.8 mm) and 1-3/8" (34.9 mm) so your frames are one of these three sizes.

I have a very lightly used 105 braze-on 8-speed triple front derailleur intended for a 52/42/30 crank. It's yours for the asking. PM me if you want it

You will need an adapter clamp of the correct diameter but any LBS should have them and Nashbar offers them at a very low cost.

I would just like to publicly give my appreciation to all the forum members who are offering their help on a daily basis to those of us who who don't have all the answers we need to keep us in the saddle, exercising our passion for biking.

In particular, I would like to thank Hillrider, who came to my aid and very generously offered his no-longer-in production FD. I note that he is a frequent contributor in the Bike Forums (as well as on other sites). It is the helpfulness and kindness such as his that make us a (biking) FAMILY.

Just to correct some measurements that I gave earlier in the opening post so that I don't perpetuate inaccuracies: The old TREK 1200 seat tube diameter was 31.8 mm and the new TREK 1500 seat tube diameter where the FD is clamped on is 34.9 mm. The solution remains identical. Many thanks, Chuck

Hello, I'm also in the same boat as original poster, looking for a 31.8 front derailleur for a 52 triple front with 8 speed cassette in rear. The original FD is a 26.8, so I can't use that. I tried to PM both Retrogrouch and Hillrider, but I need 50 posts to do that, and don't have that many yet. If anyone has an old 31.8 FD or a braze-on that I could use with an adaptor, I would greatly appreciate it!